The invention relates to rotary drill bits and in particular to bits which are used to drill holes in rock or other subsurface formations, for example to extract oil, gas or water, or in mining.
The invention relates to drill bits of the kind comprising a bit body, for connection to a drill string, a passageway for drilling fluid within the body, which passageway communicates with one or more openings in an external surface of the body, cutting elements mounted on the external surface of the body for cutting or abrading the formation, and one or more elongate fences upstanding from the external surface of the body to control the flow of fluid from said opening or openings and past said elements. The low of fluid, controlled by said fences, serves to carry away cuttings and chippings removed from the formation by the cutting elements and also to cool the elements and the formation being drilled.
The cutting elements may be polycrystalline diamond compacts and may be arranged in rows alongside wall portions of channels or grooves provided in the surface of the bit. The fences are spaced from the cutting elements and may extend substantially parallel to the channels or grooves so as to control the flow of fluid along the blades and past the cutting elements.
It is desirable that the free edge of each fence remote from the bit surface should sealingly engage the formation so as to prevent flow of fluid across the fence, since this would reduce the flow of fluid past the cutting elements. However, the formation being drilled will usually be of varying hardness and this affects the quality of the seal between the fence and the formation. During drilling through formation of a fairly consistent hardness the free edge of the fence engaging the formation will be worn away to an extent depending on the depth of cut of the cutting elements and will form an effective seal with the formation. However, if the drill bit then enters formation of greater hardness, the depth of cut of the cutting elements will immediately be reduced with the effect that the fence will no longer sealingly engage the formation. Flow can then occur across the fence, leading to inefficient cooling and carrying away of cuttings and chippings. Furthermore, the rubbing engagement between the fences and the formation provides significant resistance to the rotation of the drill.
The present invention sets out to provide an improved form of rotary drill bit in which these disadvantages may be overcome.